There are several standards for inspecting the integrity of welded or riveted, atmospheric pressure, aboveground storage tanks (ASTs) after they have been placed in service. API 653 (API 12R1 is similar to API 653 but is designed for production tanks) covers the maintenance inspection, repair, alteration, relocation, and reconstruction of such tanks. It is a performance-based inspection with the time between inspections being 10 years or more for out-of-service inspections and 5 years or less for in-service inspections. The scope of this API publication is limited to the tank foundation, bottom, shell, structure, roof, attached appurtenances, and nozzles to the face of the first flange, first threaded joint, or first welding-end connection. While it can be used for inspecting shop-fabricated tanks, it is mainly intended for field-erected ASTs. It is also used for many of the military's large, bulk underground storage tanks. In September 2000, the Steel Tank Institute (STI SP001) published a standard for inspection and repair of shop-fabricated steel tanks. The STI standard addresses double wall tanks and tanks with integral secondary containment pans as well as horizontal tanks; none of these tanks are within the scope of API 653. This standard includes a risk-based approach to inspections, where tanks with the most risk requiring more frequent inspections. The risk-based approach is a function of the size, containment, release prevention and detection, and corrosion history of the tank.
In 1988, the U.S. Environmental Protection Agency (EPA) Code of Federal Regulations (CFR) 40 CFR Part 280 and 40 CFR Part 112 mandated industry standard inspections on tanks and piping that have the potential of impacting the environment as the result of a product release due to a leak or a tank or pipe failure. Each state has implemented this regulation with the EPA standards establishing the minimum requirements. Large, bulk underground and all aboveground storage tanks were excluded from the integrity parts of the regulation. Until recently, only a few states regulated the inspection of these large tanks for integrity. The Spills Prevention Controls and Countermeasures Program (SPCC) generally controls the inspection of petroleum facilities containing ASTs or bulk USTs. Recently, the guidelines for periodic inspection of these large tanks have become mandatory. The petroleum industry has been performing inspections on their tanks for many years, because it is the criterion by which a facility is judged when tank release or tank failure incidents occur.
API Recommended Practice 580 describes the elements of a risk-based approach to an inspection program. It provides the guidance for developing, implementing, and maintaining a risk-based inspection (RBI) program. The guidelines include the means for assessing the program and its plan, while emphasizing safe and reliable operations. The ultimate goal of an internal inspection is the safety and reliability of the operating facilities. A risk-based approach, which takes into account the probability of a failure and the consequence of a failure, can be used to set better intervals between inspections. This approach acknowledges that it is important to focus the highest efforts and resources on addressing maintenance and repairs on those facilities needing it most. By focusing these efforts where they are needed most, more problems will be found earlier and the facilities will be operated safer and less expensively. A risk-based approach also saves money and a permits better use of the operational facilities, because they do not need to be taken out of service before it is necessary. The routine time interval, which has been the practice, can be very costly and may result in less than optimal maintenance and repair. A risk-based approach will better prioritize and manage the tank inspection program. The method and apparatuses of the present invention describe such an approach for determining the time interval between inspections, including an estimate of how long a scheduled inspection can be postponed. The method and apparatuses of the present invention are very measurement oriented, and, as such, will achieve better acceptance by the regulators and the facility operators.
As stated above, there are basic two types of inspections: an in-service and an out-of-service. The in-service inspection requires an inspection of the external parts of the tank, including the tank shell and the chime, while an out-of-service inspection requires both internal and external inspection, including the tank floor. In general, API 653 and most regulatory agencies require an out-of-service inspection every 10 years unless the tank is in good shape, the corrosion rate is low, and the minimum required thickness of the tank floor will not be exceeded in 10 years. An out-of-service inspection is very expensive, not only because of the inspection itself, but the loss of the tank for operations during the inspection, repairs, and maintenance activities. An in-tank inspection, which is normally performed every five years, is less expensive, because the inspection can be performed on those parts of the tank that are visually and easily accessible.
API and others have developed a formula for quantifying the Minimum Remaining Thickness (MRT) of the bottom of the tank after a floor inspection and any necessary repairs have been made:MRT={Minimum of RTbc or RTip}−Or(StPr+UPr)
where
MRT: Minimum remaining thickness at the next inspection
Or: Interval to the next inspection in years
UPr: Underside corrosion rate before repairs
StPr: Internal corrosion rate before repairs
RTbc: Minimum remaining thickness from underside corrosion after repairs
RTip: Minimum remaining thickness from internal corrosion after repairs
Thus, if we assume that we want the floor to have a minimum thickness of at least 0.1 in. (MRT=0.1 in.), we can compute the interval between inspections, or if we measure or know UPr, StPr, RTbc, and RTip. If UPr=0.135 in./year and StPr=0.0034 in./year based on the corrosion rate determined from the previous inspection, and if RTbc=0.135 in. and RTip=0.170 in after repairs have been made, then Or=6.03=6 years. These thickness and corrosion data can be obtained from UT floor thickness measurements and magnetic flux floor measurements. The present method and apparatuses make a very good estimate of this thickness during the Extension Time.
The method and apparatuses of the present invention is motivated by the need to better manage out-of-service tank inspection, maintenance, and repair activities in aboveground storage tanks and large, bulk underground bulk tanks, and pertains to all types of products/liquids and all types of tanks (e.g. production, refined petroleum, and chemical tanks). The method and apparatuses are aimed at supporting and help better managing planned annual facility inspection, repair, maintenance, and repair programs. In general, each tank facility plans to perform an inspection on one or more tanks each year so that all tanks undergo an inspection each 10 years. Longer inspection intervals are possible (e.g. 20 years) based on the corrosion rate determined in an API 653 inspection. In many facilities and for most regulatory requirements, the selection of the tanks to undergo an out-of-tank inspection is done on a time basis. Risk-based inspection assessments are used to better focus, prioritize, and manage the efforts on tank inspection programs so that the focus is on those areas of tank integrity with the highest risk. A risk-based approach to tank inspection permits better and more informed decisions about the need, priority, and schedule for inspections. As stated in API 580, a risk-based inspection needs to estimate both the likelihood of a failure and the consequence of the failure. The former is difficult to estimate because of the limited access to the inside of the tank, particularly the floor of the tank, between out-of-service inspections. Corrosion to the bottom and top of the tank floor can lead to failure and leakage. The latter is easier to estimate because one can more easily estimate the maintenance and repair costs, costs due to the loss of operations, and the cost due to cleanup/remediation if a tank fails or leaks. However, the necessary data needed to estimate the likelihood of a failure is difficult to obtain. The main problem is determination of the condition of the tank floor. This information is difficult to obtain because access to the inside of the tank is limited. If it can be determined that the tank floor is in good condition, i.e. adequate thickness, no holes or cracks, and a low-rate of corrosion, then the time between inspections can be increased and more focus can be placed on those tanks in a poorer condition.
If a facilities manager plans to take 4 tanks out of service each year, the important questions to ask are:    (1) Which tanks in the facility should be inspected first, because they are in most need of maintenance and/or repair?    (2) Does each of these tanks selected for an out-of-service inspection need to be done at this time or can the inspection be postponed because the tank is in good shape?    (3) Given that a number of tanks are scheduled for an inspection, which tanks should be inspected first?
The proposed method and apparatuses give the tank owner and operator a method for determining the priority and best schedule for tank maintenance. It is very expensive to take a tank out of service and to perform an inspection. The costs include the cost of the inspection itself and the lost operational benefit of the tank. This is especially true for small tank facilities where one or more products cannot be serviced. Once the out-of-service inspection is complete and the tank it returned to service, the tank must have sufficient floor thickness to avoid structural failure until the next inspection. The minimum thickness of the floor between now and the next inspection needs to be greater than 0.1 in. for tank bottoms and foundations with no means for detection and containment of a bottom leak. The minimum thickness is less (0.05 in.) for tanks with bottom and foundation designs with a means to provide detection and containment of a bottom leak if one were to occur.
Regular in-service inspections are performed on most tanks. These inspections are typically conducted every 5 years and address the condition of the accessible portions of the tank. Visual inspection is a very important part of this process. UT measurements of the thickness of the shell are routinely made to insure the tank has sufficient wall thickness (i.e. strength) to support the product. Unfortunately, most problems occur in the tank floor where visual access is not possible without tanking the tank out of service, removing the product from the tank, and cleaning the tank. As a consequence, the interval between tank inspections is typically set based on a schedule and/or the rate of corrosion estimated from a previous tank inspection. More recently, risk assessment procedures have also been developed to determine this interval. Typically, the time interval between inspections is 10 years although longer intervals may be possible for tanks in good shape. The basic internal inspection procedure (API 653, API12R, or STI SP001) is designed to insure that the structure is in good shape (i.e. not corroding) with the walls, floor, and appurtenances having adequate thickness to structurally support tank operations until the next inspection. As stated above, API 580 has established the guidelines for implementing a tank inspection with the goal of establishing meaningful and cost-effective intervals for inspections. However, they have not specified methods or acceptable approaches. They acknowledge that tanks that have the potential for being in poor shape should be inspected more frequently than tanks in good shape. The difficulty has been to meaningfully assess the condition of the tank and to meaningfully set a safe inspection interval.
There have been a number of approaches for assessing the condition of the tank, for better prioritizing which tanks should be inspected first, and for safely and reliably extending the time between inspections.
The main methods used to justify extending the time between inspections have been AE methods. For example, a number of methods using the AE inspection method called TANKPAC™ by Physical Acoustics have been developed and evaluated. Up to 4 years are possible with these methods. Generally, 12 AE sensors are mounted on the external wall of an AST, and the data is collected and analyzed after a 24-h waiting period for the tank to become acoustically quiet. This method is expensive to use and requires a high degree of technician skill to obtain accurate results. Furthermore, AE methods do not measure floor thickness and are not reliable as leak detection methods. In addition, these methods are not generally accepted in all circles. However, extensive field evaluations where full out-of-service internal inspections have followed such AE measurements, it is clear when certain results are obtained there is a very high probability that the tank is in good shape. If combined with a leak detection method and/or a local measurement of floor thickness, a strong basis for extending the interval between inspections is provided. These methods have been extensively used in many countries of the world. Other types of that assess the rate of corrosion from LRUT sensors placed on the outside wall floor have also been used. The main issue is that AE methods do not measure floor thickness and as a consequence, corrosion rates and floor thickness cannot be accurate evaluated.
However, the method of the present invention can utilize the strongest part of the test (i.e. when the AE system indicates no damage), the spatial distribution of the corrosion activity, and the leak detection capability as an element in the method that benefits greatly from actual floor measurements.
Loo [1999] reported on a study of 148 aboveground storage tanks inspected using an AE method (TANKPAC™ produced by Physical Acoustics) of assessing the corrosion activity in the floor of an aboveground storage tank while in-service. The AE results for each of these 148 tanks were compared to the results of an internal tank floor inspection performed as part of an out-of-service inspection. Of the 148 tanks, 33 were crude tanks and 115 were refined product tanks. The results were summarized in FIG. 2 of Loo's paper. The results of the internal inspections (i.e., the actual or true condition of the tank) were reported in terms of four categories (FU1, FU2, FU3, and FU4). The results of the AE tests, which were reported in terms of five corrosion grades from A to E (as defined below), were compared to the out-of-service inspection results. The definitions of the AE Test Results and the Out-of-Service Internal Inspections are given below:
AE Test ResultsMaintenance and RepairA: Very minorNo maintenance necessaryB: MinorNo maintenance necessaryC: IntermediateSome maintenance is neededD: ActiveGive priority in maintenance scheduleE: Highly activeGive highest priority in maintenance scheduleOut-of-Service Internal Inspection ResultsFU1: No damage/No repair(A)FU2: Minor damage/No repair(B, some C)FU3: Damage/Some repair(D, some C)FU4: Damage/Major repair/New floor(E)As will be described below, there is some uncertainty on how to compare the results of the 5 AE Test Results with the four Internal Inspections, mainly with respect to Grade C and FU2 and FU3.
As described by Loo, the AE method is intended to distinguish good tanks from bad tanks and is a really considered a sorting technique as applied. Table 1 summarizes the results obtained from FIG. 2 of Loo. The table illustrates some very important conclusions about (1) the overall condition of the tanks in the population and (2) the overall reliability of the AE method. Depending on the actual results of the AE test, it can be very reliable in supporting the tank assessment either as applied by Loo or as applied in the present invention. However, in general, our assessment of these results concludes that the AE method leads to correct decisions about the condition of the tank floor only 76.7% of the time with a probability of false alarm of 14.5% and a probability of missed detection of 8.8%. Even the correct decisions are not easy to determine because false alarms and missed detections happens for all grades except A.
When the results of the AE test indicate an A grade, there is a high level confidence that the rate of corrosion of the tank is low and no maintenance or repair of the tank is required. This accounted for 30.5% of the tanks evaluated, i.e., about 1 in 3 tanks tested. Furthermore, when the results of the AE test indicate a B grade, there is also a high level of confidence about the rate of corrosion being low with no maintenance or repair required, but at least 4, and up to 6, of the 41 tests results classified as a B grade are actually missed detections. It is difficult to determine how to interpret the C grade and the FU3 test results, because as defined the FU3 should be equated to a D grade, but the results tend to show many of the FU3 tests are C grades. In general, for our analyses in Table 1, we have assumed about half of the FU3 test results are missed detections and half would be included with the FU1,2 test results as we might expect from the definitions.
Five of the D and E grades were actually assessed as FU1, 2 tanks and were judged to be in very good condition, i.e., false alarms. Ten of the tests graded as a B were actually assessed as FU3 and FU4 where damage has occurred, i.e., missed detections. Thus, how the AE test results are used in the method of the present invention is very dependent on the actual results of the AE test. More reliability can be assigned to the AE test results when a local UT floor thickness measurement is used to help interpret the results. Also, more reliability can be assumed if a more advanced signal processing method is used to determine the grade.
As stated above, strong statements can be made when an A or B grade is determined, particularly for an A grade. This is not the case for D and E grades, because there are almost as many tanks in need of repair and maintenance as prescribed by FU4 that receive a B or C grade vice a D or E grade. If the AE test results indicates a problem, you would be correct only 38.1% of the time. Similarly, if the AE results were A, B, or C, you would miss 9 of the 123 tanks (7.3%) in need of serious maintenance and repair and possibility more if the tanks with a C grade need some repair. Thus, we would consider the results of a previous API 653 inspection to be more reliable than a current AE inspection if the results of the AE test were D or E.
Some general conclusions about the condition tanks in general can be made from Table 1, which can support the overall method. First, 64.2% of the tanks tested need little or no maintenance or repair. Thus, there is almost a 2 in 3 chance that any tank that passes a Leak Test is in good shape. Second, 14.2% of the tanks tested need significant maintenance and repair. Thus, a Leak Detection Test will correctly identify 78.4% of the tank conditions. This leaves 21.6% as uncertain with more information needed to ascertain their true condition. In general, we would expect that almost all of these 21.6% of the tanks would pass a Leak Detection Test even though they still need some repair and/or some maintenance. Thus, some measurement of the tank floor condition is needed in addition to the AE test.
TABLE 1Summary of the AE Corrosion Activity Tests [Source: Loo, (1999)].AE% ofNumberCum %GradeTanksof Tanksof TanksFU1/2FU3FU4FU1/2FU3FU4A30.5%4530.5%100.0%0.0%0.0%100.0%450045B27.5%4158.0%76.0%14.0%10.0%100.0%316441C25.0%3783.0%38.5%48.5%13.0%100.0%1418537D7.5%1190.5%18.5%45.0%36.5%100.0%25411E9.5%14100.0%21.0%21.0%58.0%100.0%33814100.0%148500.0%95322114864.2%21.6%14.2%100.0%AE GradeCorrect DecisionIncorrect DecisionFalse AlarmsMissed DetectionsA45100.0%00.0%00.0%00.0%B3175.6%1024.4%1024.4%00.0%C2362.2%1437.8%924.3%513.5%D6.559.1%540.9%322.7%218.2%E857.1%642.9%00.0%642.9%11476.7%3523.3%221376.7%23.3%62.3%37.7%
Cole and Gautrey [2002] described history of the AE method used in the Loo study (TANKPAC™ produced by Physical Acoustics) and included additional data and illustrations of the use of the method for ascertaining the condition of a tank and whether or not the time between scheduled internal inspections can be extended. Their FIG. 10 increased the number of tanks used in Table 1 from 148 to 157; the results were very similar. In their FIG. 11, they reported the results of a similar study by the French Institute of Petroleum for a sample population of 78 tanks with very similar results. Table 2 compares the results from Loo, Cole and Gautrey, and the French Institute of Petroleum and shows that they are very similar. The main conclusions hold: (1) tanks with AE reported grades of A and B (FU1,2) show very little or some maintenance and repair required, (2) tanks with AE reported grade of D and E showed large damage (FU4) with a high degree of maintenance and repair needed, (3) a small number of false alarms in which AE reported grades of D and E were actually in good shape (FU1,2) or not in very bad shape (FU3), and (4) no missed detections in which tanks in very bad shape (FU4) were reported in good shape. The strongest statements that can be made about the AE test is that if a test results in a grade of A or B, it should be in good shape. Conversely, if a tank is reported with a grade of D or E, it should be treated as such even though about half of the tanks in this category were actually in moderate to good shape.
TABLE 2Summary of the AE Corrosion Activity Test Results from Three Sources.Loo (1999):French Institute of PetroleumCole and GautreyAE148 Tanks(2002): 78 Tanks(2002): 157 TanksGradeFU1/2FU3FU4FU1/2FU3FU4FU1/2FU3FU4A100.0%0.0%0.0%100.0%0.0%0.0%100.0%0.0%0.0%B76.0%14.0%10.0%89.0%11.0%0.0%80.5%11.5%8.0%C38.5%48.5%13.0%22.0%18.0%60.0%36.0%40.0%24.0%D18.5%45.0%36.5%19.0%29.5%51.5%15.0%45.0%40.0%E21.0%21.0%58.0%3.0%14.0%92.0%12.0%26.0%62.0%
Mejia, Hay, Mustafa, and Santa Fe [2009] described a method of using AE to extend the time between scheduled inspections. Their Tables 2 and 3 summarize the application of their method, which combines the Overall Corrosion Grade A through E with a Potential Leak Detection Grade. The Potential Leak Detection Grade indicates areas of highly concentrated clusters of AE events, where 5 indicates very high potential and 1 indicates very low potential. Unlike the present method of this invention, this method does not specifically test for a leak. Table 3 summarizes their results. The schedule for an internal inspection can be postpone up to 4 years for A, B and 1, 2 grades, up to 2 years for C, D, E and 1 and most 2 grades, as well as A, B and 1, 3, 4 grades. A postponement of 0.5 and 1 year is possible for all but D, E and 5 grades. The method of the present invention allows extensions up to 5 years and has more reliability in the decision process, because the present method is based on whether or not a tank is leaking as determined from a Leak Detection Test and what the thickness and corrosion rate of the floor is from actual measurements. The data from the AE test as included in the method of the present invention can also be used to test the tank floor for leaks.
TABLE 3Summary of the AE Corrosion Activity Test Interpretation in terms of thenumber of years extension for a potential leak grade and a Corrosion grade[Source: Mejia, Hay, Mustafa, and Santa Fe (2009)].PotentialLeakOverall Corrosion GradeGradeABCDE1IIIIN/AN/A2IIIIN/AN/A3IIIIIII*III N/A4IIIIIIIIIVIV5IIIIIIIVIVIVI: Extension Time Interval = 4 yearsII: Extension Time Interval = 2 yearsIII: Extension Time Interval = 1 yearsIV: Extension Time Interval = 0.5 yearsPotentialLeakOverall Corrosion GradeGradeABCDE1442N/AN/A2442N/AN/A322ScheduleScheduleN/A42ScheduleScheduleScheduleSchedule5ScheduleScheduleScheduleScheduleSchedule*or 6 months or 1 Year